The biconical-dipole antenna is intended for use in determining the electric field intensity as a function of frequency at various short distances from an emanating source, such as a computer, or in environments where weak electromagnetic fields may exist. The antenna is calibrated in such a way that the measured output voltage across a specified resistive load (usually 50 ohms) yields the incident electric field intensity in volts per meter by way of a calibration curve. The calibration curve, called an antenna factor curve, usually expresses the ratio of the incident electric field intensity E in volts per meter to the output voltage V.sub.o (that is, the AFE) in decibels. The AFE thus enables the determination of E by the measurement of V.sub.o. Mathematically stated, EQU E(V/m)=V.sub.o (V).times.AFE(m.sup.-1), (1)
from which by taking logarithms of each side times 20, one finds EQU 20 log E(V/m)=20 log V.sub.o (V)+20 log AFE(m.sup.-1), (2)
which, expressed in decibels (dB), reads EQU E(dB V/m)=V.sub.o (dB V)+AFE(dB m.sup.-1), (3)
In the above equations (1) through (3), the units of measurement are included inside the parentheses along with the decibel (dB) notation. The calibration curve gives the AFE in decibels for any frequency in the working range of the antenna. The accuracy of the antenna factor calibration is very important especially when limits are imposed on the allowable level of the incident field being measured.
A typical biconical-dipole antenna is specified in military standards bulletin MIL-STD 461. This antenna is intended to function from 20 MHz to 350 MHz. The calibration curve for this antenna exhibits a sharp peak, or suck-out, at 290 megahertz, indicating a sharp loss in sensitivity in the antenna. The exact frequency of this resonance will generally vary somewhat from one antenna to another of the same design because of manufacturing tolerances and/or proximity to other objects in the immediate vicinity. The greater the antenna factor becomes, the less becomes the sensitivity, or response, to a given signal. The sharpness of this so-called suck-out is very disconcerting to a user trying to determine the intensity of an existing field, or the existence of any field at all, at or near this frequency. The present invention is designed to overcome or minimize this loss of sensitivity.